Probably, since the main art for the game shows Mario on a grown Yoshi. But now that the baby Yoshi's have special powers, will you want them to grow? Maybe the adults will have 'em too, what do I know.---All's fair in love and war, kid.

^ Could just be for the demo, maybe they didn't finish adult Yoshi's yet. All it takes is changing a single variable to keep them from growing into adults for a demo.---All's fair in love and war, kid.